The present invention relates to an electron source and a process for manufacturing the same. More specifically, the present invention relates to a cold cathode electron source applicable to a back light for a liquid crystal device, various types of light sources, and a flat panel display for a computer or television, and also relates to a process for manufacturing the cold cathode electron source.
In a conventional electron tube, such as a cathode ray tube (CRT), a hot cathode has been used as the electron source. A hot cathode is a cathode which utilizes thermionic emission to produce electrons. Thermionic emission is a mechanism in which a cathode material is heated to 1500-2700 K to apply energy higher than the work function to free electrons in the conduction band, whereby electrons are emitted beyond the potential barrier of the surface of the cathode material. Examples of such material include pure metals and oxides. The material most frequently used today is a sintered-type material prepared by press-sintering a mixture of a barium (Ba)-based compound (e.g., 5BaOxc2x72Al2O3xc2x7CaO) and tungsten (W) powder or an impregnated-type material prepared by impregnating porous W with a Ba-based compound in a molten state. The two types of materials have an advantage in that the electron emission density is high and, in addition, the discharge of gas generated during vacuum evacuation is small and the cathode material can be re-activated even when the cathode is exposed to the atmosphere because barium aluminate is contained in the material.
Besides thermionic emission, electron emission includes field emission, secondary electron emission, photoelectron emission and so on.
A cold cathode is a cathode which can emit electrons by field emission. In a field emission type of cathode, a high electric field (109 V/m) is applied in the vicinity of the surface of the cathode material to lower the potential barrier of the surface (which is the so-called xe2x80x9ctunneling effectxe2x80x9d), whereby electrons are emitted. This type of cathode is called a xe2x80x9ccold cathodexe2x80x9d because it does not require the heating of the cathode material like a hot cathode.
The current-voltage characteristic of a field emission type of cold cathode can be approximated in accordance with the Fowler-Nordheim equation. The electron-emitting section of the cold cathode is designed to have such a structure that the field enhancement factor is increased (i.e., a needle-like structure), since a high electric field is applied to the electron-emitting section while keeping the insulation state. An early type of cold cathode has a diode type structure formed by electrolytic polishing of a needle-shaped single crystal. In recent days, however, a remarkable progress has been made in the technique for manufacturing field emission type of electron sources (i.e., field emitter arrays) which can emit electrons in a high electric field by virtue of the development of micromachining techniques as used in the field of integrated circuits or thin films. In particular, a field emission type of cold cathode with a microstructure has been manufactured successfully. Such a field emission type of cold cathode is the most essential electron emitting element among the essential elements of a triode type micro-electron tube or micro-electron gun. A field emission type of cold cathode with a microstructure has an advantage in that a higher current density can be provided to a hot cathode.
A field emission display (FED) with a cold cathode is expected to be applicable to a self-emitting type of flat panel display. Under these situations, research and development of a field emission type of electron source have been aggressively made.
FIG. 14 is a sectional view showing the basic configuration of a prior art FED.
xe2x80x9cAs shown in FIG. 14, the FED consists mainly of a back plate 18 which effects the emission of electrons; a face plate 10 in which fluorescence-emission is effected from a luminant 11 by irradiation of an electron beam 2 from the back plate 18, whereby an image is displayed thereon; side walls 19 for vacuum-sealing a space between the back plate 18 and the face plate 10; and spacers 15 for supporting the gap between the back plate 18 and the face plate 10 and maintaining the strength of the structure of the FED against atmospheric pressure. The back plate 18 is provided with a gate electrode 14 via an insulator 16. The gate electrode 14 is used for the application of an electric field to a cold cathode 13. The cathode lines and the gate lines usually form together the X-Y matrix for addressing of pixels. When the gap between the back plate 18 and the face plate 10 is wide, a focusing electrode 17 may be required for focusing the electron beam 2. Since the FED is a type of vacuum device like a CRT or a vacuum tube, a micropump called a xe2x80x9cgetterxe2x80x9d is disposed in the vacuum space between the back plate 18 and the face plate 10 for the purpose of maintaining the vacuum level of the vacuum space after the vacuum-sealing thereof. The getter includes an evaporation type and a non-evaporation type. An evaporation type getter generates a fresh and active gettering surface thereon by heating evaporation or the like, on which evacuation is achieved by means of the chemical adsorption of gas onto the gettering surface. In a non-evaporation type getter, gas chemically adsorbed on the gettering surface (which has been activated by heating to a high temperature) is diffused into the getter material, whereby evacuation is achieved. If the non-evaporation type getter is made of the same material, its evacuation ability depends on the volume and the surface area thereof, and becomes higher as the volume and surface area become larger.xe2x80x9d
In the field emission, the amount of emission current may vary 2 to 3 times for 2 to 3% of change in electric field. Therefore, when the field emission is applied to a FED, it is required to introduce a control layer, such as a ballast resistor layer.
On the other hand, there has been reported a laminate of a metal plate having through-holes and a control electrode for an electron beam to manufacture a hot cathode electron source (Japanese Patent No. 2558993).
It has been proposed to use a ceramic substrate laminate for the formation of ribs in a plasma display (Japanese Patent Application Laid-open No. 3-45565).
As the material of a field emission type of electron source for a FED, various kinds of materials have been known. Recently, a carbon nanotube (CNT) has attracted much attention as the electron emission material.
A carbon nanotube was originally developed by Iijima et al (S. Iijima, Nature, 354, 56, 1991). The carbon nanotube has a nested structure of cylindrically wound graphite layers, of which tip has a diameter of about 10 nm. The carbon nanotube is believed to be a very excellent material as an electron source array due to its properties such as high resistance against oxidation and ion bombardment. There are experimental reports on the field emission from carbon nanotubes by the research groups of R. E. Smalley et al. (A. G. Rinzler, Science, 269, 1550, 1995) and W. A. de Heer et al. (W. A. de Heer, Science, 270, 1179, 1995). The carbon nanotubes used in these experiments were casted on a metal thin plate.
The carbon nanotube has a structure having a high aspect ratio. Therefore, the electron source using carbon nanotubes is assumed to exert a higher electron emission efficiency when the carbon nanotubes are oriented in the direction of the electric field applied.
As a known electron emission element using oriented carbon nanotubes, there is mentioned a triode type one comprising carbon nanotubes which are selectively grown in small holes provided in an anode oxide film (Japanese Patent Application Laid-open No. 10-12124). In the electron emission device, the variation in properties of the electron source in each pixel is reduced and the stability of current intensity per pixel is improved.
It has been also proposed to orient carbon nanotubes on SiC crystals under vacuum (Japanese Patent Application Laid-open No. 10-265208).
However, in the fabrication process of the conventional FED, a back plate is prepared by forming a ballast resistors, cathode lines, gate lines and an insulating layer successively. When the formation is to be performed using a vacuum apparatus, the process becomes more complicated. In a FED, the gap between the back plate and the face plate is invariant even if its screen size is increased. Therefore, when it is intended to manufacture a large-screen FED, it is difficult to maintain the vacuum level of the vacuum space between the face and back plates simply by providing getters in the vicinity of the side walls and the corners.
The formation of ribs in a plasma display mentioned above is made only for the convenient formation of the configuration of the display, and does not take electrical wiring or maintenance of the vacuum level into account.
In the lamination of metal plates mentioned above, it is difficult to put the electrodes closer to each other for the reason that it is needed to keep a some certain distance between the electrodes for maintaining the insulated state. It is also difficult to form the X-Y matrix as an extraction electrode of the cathode.
In the conventional carbon nanotube cold cathode electron source with small holes provided on an anode oxide film, there is a problem of the damage to Al contained in the substrate due to the high temperature condition employed for the production of the carbon nanotubes. On the other hand, when it is intended to orient the carbon nanotubes on SiC crystals, a special apparatus is needed, since the formation and patterning of the carbon nanotubes are performed in a vacuum. Moreover, due to the high density of the carbon nanotubes on the crystals, the level of field enhancement determined depending on the structural factor of the carbon nanotubes may become small. Thus, it is hard to utilize such an advantageous property of a carbon nanotube that it can emit electrons at a low voltage.
The present invention has been made in view of these circumstances. Accordingly, the object of the present invention is to provide the low-cost manufacture of a device with a cold cathode electron source which can perform vacuum evacuation and maintain the vacuum level and can also provide a high emission current density at a low voltage.
Accordingly, the present invention provides an electron source comprising a first layer having at least a cathode electrode and an emitter section, a second layer having a control electrode which is disposed on the first layer and a spacer or spacers disposed on the second layer; the first and second layers being laminated to form a sheet-like structure, and the laminated structure being calcined to form an integral structure having the spacers thereon.
By providing the constituent elements on a plurality of uncalcined ceramic sheets, respectively, and laminating and calcining the sheets to form an integral structure, it becomes possible to form a laminate structure without the need of multilayer formation technique using a vacuum apparatus for layer-formation and a printing apparatus.
In the electron source, a plurality of through-holes may be formed in the first layer and a room for gettering may be provided on the backside of the first layer. These contrivances enable to maintain the vacuum level of the vacuum sealing tube and insulate from the gate lines.
In a preferred embodiment of the electron source, the emitter section may comprise a material capable of emitting electrons at a field strength of 10 V/pm or lower, or the emitter section may comprise a carbon nanotube.
The present invention also provides a process for manufacturing an electron source, the process comprising sucking a material containing carbon nanotubes dispersed therein into through-holes provided on a flat plate to cause the orientation of the carbon nanotubes in the material in the axis direction of the through-holes.
The present invention further provides a process for manufacturing an electron source, the process comprising press-charging a material containing carbon nanotubes dispersed therein into through-holes provided on a flat plate to cause the orientation of the carbon nanotube in the material in the axis direction of the through-holes.
The charge of the material containing the carbon nanotubes into the through-holes by suction or application of pressure enables to cause the orientation of the carbon nanotubes in the through-holes without the need of any orientation process or processing.
In a preferred embodiment of the process, the tip of each of the through-holes may be tapered, or the material having carbon nanotubes dispersed therein may be a highly resistive material.
This specification,includes part or all of the contents as disclosed in the specification and/or drawings of Japanese Application No. 2000-123180, which is a priority document of the present application.